Debugger: Move some debugger specific bits out of debuggermainwindow.*

A step towards making the perspective handling independent of the
debugger plugin.

Change-Id: Ic07f4b34d44c48f16a494ba7f470e0a34d3d56a3
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/debugger/debuggermainwindow.h

@@ -140,6 +140,4 @@ private:
     class DebuggerMainWindowPrivate *d = nullptr;
 };
 
-DEBUGGER_EXPORT QWidget *createModeWindow(const Core::Id &mode, QWidget *);
-
 } // Utils